Plasma etching systems have been used now for several years in the semiconductor manufacturing industry as an alternative material-removal method to the earlier so-called wet etching processes. In these earlier wet etching processes, material removal from a semiconductor substrate or the like was accomplished by the chemical interaction between a liquid etching compound or mixture in direct physical contact with the material to be removed. On the other hand, plasma etching systems rely upon molecular/atomic (ionic) particle acceleration and bombardment of a material to be removed. These particles will typically be certain ion species which may be made available from a plasma reaction in a gas reaction chamber which is operated under carefully controlled conditions of temperature and pressure.
Plasma etching systems are generally well known in the art and are described in the following publications incorporated herein by reference:
(1) Plasma Etching in Semiconductor Fabrication by Russel A. Morgan, Elsevier Press, Copyright 1985.
(2) Glow Discharge Processes by Brian Chapman, Wiley & Co., Copyright 1980.
(3) Materials Research Society Symposia Proceedings, Volume 68 ("Plasma Processing") by J. W. Coburn, R. A. Gottschio and D. W. Hess.
One type of plasma gas reaction chamber contains charged electrodes between which electrons are accelerated to collide with certain gas molecules to thereby free up the ions species from these molecules which can then be accelerated in the presence of an electrical field and caused to bombard the surface of the semiconductor material desired to be removed. These plasma etching systems thus have been referred to as "dry" etching systems in contrast to the earlier used wet etching systems and have many advantages over these earlier wet etching systems as is well-known in the art.
Standard plasma etching equipment used, for example, in the semiconductor industry for etching semiconductor wafers includes, among other things, a plasma chamber surrounding an electrode for receiving a semiconductor wafer in direct contact with the electrode surface. This electrode provides the physical support for the wafer and electrical potential necessary for the acceleration of ions as mentioned above into bombardment with the semiconductor wafer or substrate. The electrode also provides an integral part of the plasma etching apparatus necessary for cooling the wafer during the plasma etching process. In addition, this cooling is necessary to prevent degradation of the etch mask as well as to consistently control polymer and inorganic depositions which determine profiles of the etched features. The etch rate of a silicon wafer in a plasma etching system can be selectively controlled by controlling the temperature of the wafer being etched. Also, the etch rate of a photoresist polymer used as an etch mask on the wafer goes up significantly with an increase in temperature, so here is another reason for maintaining good cooling and heat transfer away from the wafer.
In order to provide the necessary high vacuum environment in the gas reaction chamber required for plasma etching, it is necessary to pump the front side of the wafer down to high vacuum levels of pressure so that typically the front or top side of the wafer in the plasma chamber 10 will be at a pressure in the range of 0 to 1.0 Torr and the back or lower side of the wafer will be pumped down to a partial vacuum in the range of 1.0 to 10.0 Torr. However, since the wafer and electrode are not normally perfectly flat, there will not be a 100% surface contact between the wafer and the supporting electrode. This characteristic in turn serves to create high vacuum voids in certain non-contact regions between the wafer and electrodes which, if not filled with certain gas molecules, will provide a poor heat transfer path for conducting heat away from the wafer and to the electrode. As will be described in further detail below, it is imperative to provide good cooling of the wafer and good heat transfer therefrom to the underlying supporting metal electrode.
To correct this latter problem of poor heat transfer between the wafer being etched and the underlying supporting metal electrode which would otherwise be created by high vacuum voids between these two components, so called "helium back side cooling" systems have been proposed and designed wherein helium gas is used to fill these high vacuum voids between the wafer and supporting electrode.
In the past, plasma etching equipment has used essentially two different designs for providing such helium back side cooling to the side of the semiconductor wafer in direct contact with the supporting electrode. The first of these designs employs a single O-ring seal between the respective peripheries of the wafer and electrode, and a gas supply line is connected between a central opening in the electrode and a source of cooling gas for introducing this gas into the closed region between wafer and electrode which is bounded peripherally by the O-ring seal. A peripheral clamp is used to secure the wafer and electrode tightly together in the vicinity of the O-ring seal.
A second design is somewhat similar to the above first design but does not use a seal between wafer and supporting electrode and instead merely relies upon the clamping action between wafer and electrode to form a barrier that will tend to keep the cooling gas such as helium out of the main plasma gas reaction chamber.
Whereas the above two prior art designs have operated satisfactorily in some respects, neither of these two designs have proven entirely adequate as a totally reliable means for keeping the helium gas from exiting the region between wafer and electrode and escaping (if only slightly) into the main gas reaction chamber. This is particularly true if the back side pressure in the chamber 10 is operated at a pressure of greater than 10 Torr.